What is X-ray Photon Correlation Spectroscopy (XPCS)?

XPCS is a novel technique that studies the slow dynamics of various equilibrium and non-equilibrium processes occuring in condensed matter systems. It is based on the generation of a speckle pattern by the scattered coherent light originating from a material where some spatial inhomogeneities are present. A speckle pattern is a diffraction limited structure factor, and is typically observed when laser light is reflected from a rough surface, or from dust particles performing Brownian motion in air. The observation of speckle patterns with hard X-rays has just been demonstrated in the last few years. This observation is only possible now because of the development of new synchrotron radiation X-ray sources that can provide sufficient coherent flux. Figure 1 shows a speckle pattern from from frozen domain walls in Cu₃Au. The image was generated with 7 keV X-rays, and measured one meter away from the sample with a X-ray sensitive CCD. Notice the large random spatial variation of the intensity. With an incoherent illumination, these spatial variation are not observed.

If the state of disorder of the system changes with time, the speckle pattern will change, thus by studying the time dependence of the scattered intensity at a fixed wavevector, one can probe the dynamics of materials in thermodynamic equilibrium or out of equilibrium. Figure 2 shows the potential of XPCS for exploring the dynamics of various processes in condensed matter system in an uncharted area of time scale and reciprocal space. The potential for discovering new types of dynamical phenomenon at atomic length scales makes it a very exciting new field of research.

Fig. 1: A speckle pattern of Cu₃Au (100) measured at room temperature after the sample had been ordered for 16 hours. This ordering was necessary to observe sufficient intensity from the weak scattering of the initially small Cu₃Au domains. The grey scale displayed shows detected counts per pixel, for a 2 minutes exposure at a standard current of 200 mA. The beam was collimated with a 7.5 micron pinhole to preserve the transverse coherence of the X-ray beam.

Figure 2: The region of wavevector-energy space covered by various techniques is indicated. XPCS would provide a unique probe in a previously inaccessible region. Thise figure is borrowed from Steve Dierker's review of XPCS in the July 1995 version of the NSLS Newsletter (see p6).

Useful XPCS links

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Récemment modifié le 21/06/23, par Eric Dufresne.